1. Field of the Invention
The present invention relates in general to a method and apparatus for acquiring information using the relative phase deviation between the phase of a received signal of an optical homodyne receiver and the phase of the optical signal of a local oscillator wherein the frequency of the signal of the local oscillator is shifted by the frequency of an auxiliary carrier oscillator and the frequency shifted optical signal is then superimposed upon the received optical signal and detected.
2. Description of the Prior Art
The present standard form of optical communication technology with intensity modulation is based on direct detection reception such as was standard in radio technology until 1930. As in radio technology, the application of heterodyne reception also gives advantages for optical communication technology. These are:
1. Higher receiver sensitivity and, thus, greater repeater section lengths; PA1 2. More favorable modulation and demodulation methods; PA1 3. The possibility of employing optical repeaters; and PA1 4. Increases in the channel capacity by means of an optical frequency division multiplex method.
Of the possible heterodyne methods, homodyne techniques together with phase keying modulation theoretically yields the greatest improvements over direct detection reception. Patricularly given homodyne reception, only the same bandwidth as required in direct detection reception is required, whereas at least three times the bandwidth is required at the optical receiver using heterodyne reception, and this causes extremely expensive IF technology where high bit rates are involved.
The difficulties in homodyne reception lie in the high demand made on the optical transmitter, or, respectively, the local oscillator with respect to its frequency stability and phase noise since the local oscillator must be synchronized and phase-locked with the transmitter. A PLL (phase locked loop) circuit serves this purpose and must have a band width of about 25 MHz assuming a line width of the transmitter or local oscillator of 1 MHz.
Using the current technology for a phase control of the local oscillator, a weak DC signal must be obtained from the detected optical power as a control variable which is superimposed by a significantly greater intensity-dependent current. The compensation of the dominating intensity-dependent DC is difficult and also a DC-coupled control circuit is undesirable because of the 1/f noise drift of the repeaters.